Methods and apparatus of tracking moving targets from air vehicles

ABSTRACT

Methods and apparatus of tracking moving targets from air vehicles are disclosed. An example system includes an air vehicle including a moving target state estimator to determine at least one of an estimated speed or an estimated location of a moving target, a tracking infrastructure to determine a detectability zone surrounding the moving target based on at least one of the estimated speed or the estimated location of the moving target, and generate a guidance reference to command the air vehicle to move towards a reference location, the reference location based on the estimated location, and a flight control system to cause the air vehicle to follow the moving target outside of the detectability zone based on the guidance reference.

RELATED APPLICATION

This patent arises from a continuation of U.S. patent application Ser.No. 15/227,707, which was filed on Aug. 3, 2016, now U.S. Pat. No.10,101,750, which claims priority to European Patent Application SerialNo. EP15382414, which was filed on Aug. 4, 2015. U.S. patent applicationSer. No. 15/227,707 and European Patent Application Serial No.EP15382414 are hereby incorporated herein by reference in theirentireties. Priority to U.S. patent application Ser. No. 15/227,707 andEuropean Patent Application Serial No. EP15382414 is hereby claimed.

TECHNICAL FIELD

This patent relates to methods and apparatus of tracking moving targetsfrom air vehicles.

BACKGROUND

Unmanned aerial vehicles (UAV's) may be involved in tasks related toIntelligence, Surveillance and Reconnaissance (ISR) missions. Thesetasks may involve following and/or tracking a moving target (which mayeither be ground-based, air-based or sea-based) from the air vehicle forpurposes such as, for example, border security, perimeter protection,wildlife monitoring, law enforcement, military operations or generalpurpose surveillance.

SUMMARY

An example method, in response to an estimated speed and an estimatedlocation of a moving target, determines a detectability zone surroundingthe moving target; and causes an air vehicle to follow the moving targetoutside of the detectability zone.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a schematic diagram of example architecture modules forpossible software/hardware implementation that can be used to implementthe examples disclosed herein.

FIG. 2 shows a schematic view of an example air vehicle tracking amoving target from behind, in which the detectability area of the airvehicle has been represented.

FIG. 3 shows a flowchart depicting an example method of tracking amoving target.

FIG. 4 shows a schematic diagram representing the positions of the airvehicle and the moving target, at block 2 of the example flowchart shownin FIG. 3.

FIG. 5 shows a schematic diagram representing the positions, course andcinematic vectors concerning blocks 3 to 5 of the example flowchartshown in FIG. 3.

FIG. 6 shows a schematic diagram representing the positions andcinematic vectors concerning blocks 6 and 7 of the example flowchartshown in FIG. 3.

DETAILED DESCRIPTION

The present disclosure relates to example methods of tracking a movingtarget from an air vehicle. The example methods consider an area oflikely detectability of the air vehicle by the moving target in such away that the air vehicle performs its tracking mission while avoidingbeing detected by the moving target.

The example methods of the present disclosure are applicable in thefield of civil and military operations involving surveillance andtracking of moving targets using air vehicles. The example methods areapplicable in the field of aeronautic engineering and, moreparticularly, in the field of electronics and automatic regulation foravionics. However, the examples disclosed herein may be applicable inany field (e.g., fields other than civil and military operations).

As shown in the example of FIG. 2, the following is a description ofexample methods of tracking a moving target 11 (e.g., a Ground MovingTarget—GMT—) from an air vehicle 10 (e.g., a UAV).

FIG. 1 shows an example schematic diagram in which different functionalblocks represent architecture modules of a software/hardwareimplementation of the example methods described in the presentdisclosure.

According to FIG. 1, an example block 100 represents an architecturemodule, namely the example Tracking Infrastructure Module 100 fortracking of the moving target 11. In the illustrated example, theTracking Infrastructure Module 100 includes two modules, e.g., anexample Air Vehicle Behavior Policies Module 102 and an example TrackingAlgorithm Module 104, which in FIG. 1 are represented by two boxes 102,104 inside the Tracking Infrastructure Module 100. Referring to FIGS. 1and 2, the Tracking Algorithm Module 104 holds the exampletracking/guidance algorithm that is used to generate guidance referencestowards which the air vehicle 10 is commanded to fly to track the movingtarget 11. In the illustrated example, the Air Vehicle Behavior PoliciesModule 102 comprises the behavioral policies of the air vehicle 10 thattake into account, among others. The example Aircraft Performance Model(APM) 108 of the air vehicle 10 in which the present example method isimplemented and a digital map of the terrain over which the air vehicle10 flies are fed into the Tracking Algorithm Module 104.

In some examples, the tracking algorithm 104 is responsible to state thegeneric problem of solving the desired relative motion of the airvehicle 10 with respect to the moving target 11 The tracking algorithmprovides the appropriate guidance references to the example FlightControl system (FC) 112 to actually exhibit such relative motion. Insome examples, the generic problem is stated in terms of severalparameters (e.g., detectability of the air vehicle 10, maximum andoptimal distances of the air vehicle 10 with respect to the movingtarget, desired altitude, desired relative location (alpha angle, aswill be described below), and also takes into account limitations statedon the air vehicle 10 performance (e.g., like maximum and minimumairspeeds, and roll, pitch and yaw angles). In some examples, the airvehicle 10 behavior policies accommodate user preferences that actuallyimpose conditions or define parameters on which the tracking algorithm104 is based (e.g., the aforementioned detectability distance, desiredaltitude, etc.).

In the illustrated example, the Tracking Infrastructure Module 100comprises three inputs and one output. As shown in the example of FIG.1, each of the inputs is respectively connected to an example MovingTarget State Estimator Module 106, to an example Aircraft PerformanceModel Module 108 and to an example Navigation System Module 110. Inother words, the Tracking Infrastructure Module 100 receives inputs fromthe Moving Target State Estimator 106, the Aircraft Performance Model108 and the Navigation System Module 110. In the illustrated example,the output of the Tracking Infrastructure Module 100 is connected to anexample Flight Control System (FC) Module 112. In other words, theTracking Infrastructure Module 100 outputs data to the Flight ControlSystem Module 112.

In some examples, the Moving Target State Estimator Module 106represents an architecture module comprising components (e.g., sensorsand other hardware/software, either onboard or not) for reliablyestimating the current (updated) state of the moving target 11. Theinput that the Moving Target State Estimator Module 106 gives to theTracking Infrastructure Module 100 is, in some examples, in the form ofa moving target state vector. For the purposes of the present method, insome examples, it is considered that the estimation of the moving target11 motion is given and/or accessible and is reliable. The moving targetstate vector may include information on position, speed and altitudes s(e.g., and sometimes their derivatives) of the moving target 11.

In the illustrated example, the Aircraft Performance Model Module 108represents an architecture module comprising the air vehicle 10performance model. Thus, the air vehicle 10 performance characteristicsare given as an input from the Aircraft Performance Model Module 108 tothe Tracking Infrastructure Module 100. In some examples, the airvehicle 10 performance is used by the tracking algorithm 104 to ensureor substantially ensure that the generated guidance references fallwithin the operational limits and/or flight envelope of the air vehicle10 (e.g., the air vehicle 10 is capable of flying in accordance with thegenerated guidance references).

In the illustrated example, the Navigation System Module 110 providesaccess to an input to the Tracking Infrastructure Module 100 of the AirVehicle State Vector. In some examples, the Navigation System Module 110is connected to air vehicle 10 sensors. In some examples, the AirVehicle State Vector given as an input from the Navigation System Module110 to the Tracking Infrastructure Module 100 comprises an air vehicle10 position, speed and altitude (e.g., and their derivatives).

In the illustrated example, the Flight Control System Module 112receives as an input the Guidance References (e.g., which in turn takeinto account the flight limitations given by the behavior policies andAircraft Performance Model—APM—) which the Tracking InfrastructureModule 100 delivers as an output. Thus, in the illustrated example, theFlight Control System (FC) 112 is in charge of and/or causes the airvehicle 10 to fly towards the references generated by the trackingalgorithm 104. In some examples, the FC System 112 is responsible forissuing signals to the air vehicle's actuators that drive the airvehicle's flight behavior. In other words, the FC System 112 causes theair vehicle 10 to fly in a particular route, trajectory, or pattern. Insome examples, the FC System 112 uses the guidance references togenerate such signals.

In some examples, guidance references for guiding the air vehicle 10 aregenerated that represent and/or are associated with a certain courseand/or speed vector for flying the air vehicle 10 and/or coordinates inthe flying field surrounding the moving target 11. In some examples, theair vehicle 10 is instructed to fly in accordance with the guidancereferences. The guidance references may be given as an input to theFlight Control System (FC) 112 and may be based in a first instance on areference location generated by the tracking algorithm 104. In someexamples, the reference location is given with respect to the movingtarget 11, e.g., the reference system used for the purpose of generatingcoordinates of the reference location is, in some examples, a referencesystem centered on the moving target 11 so the coordinates of thereference location are given as a position vector with respect to themoving target 11.

Based on the reference locations generated by the tracking algorithm104, in some examples, the tracking algorithm 104 calculates theguidance references that are given as an input to the Flight ControlSystem 112.

In some examples, the Flight Control System (FC) 112 may use threedifferent guidance references to fully determine the air vehicle motion.However, it may expose less than three to the user though. In someexamples, one of the three references defines the lateral motion of theair vehicle 10. In some examples, the other two references define thevertical motion of the air vehicle 10. Thus, some guidance referencesdefine vertical motion and other guidance references define lateralmotion.

In some examples, the guidance references may be selected among anycombination of one or more of at least: 1) a desired and/or thresholdcourse for the air vehicle 10 to fly towards the generated referencelocation; 2) a desired and/or threshold speed for the air vehicle 10(either expressed as airspeed or groundspeed), to fly towards thereference location; and/or 3) a desired and/or threshold altitude forthe air vehicle 10, for example.

As noted above, in some examples, some Flight Control Systems (FC) 112may expose less than three guidance references. For example, some FC'smay not receive as three input guidance references. For example, someFlight Control Systems may establish their own value for air vehiclespeed once they have received inputs of a desired and/or thresholdcourse and a desired and/or threshold altitude.

In some examples, a desired and/or threshold course for the air vehicle10, a desired and/or threshold altitude for the air vehicle 10 and adesired and/or threshold speed for the air vehicle 10 are firstcalculated in terms of the reference location generated by the trackingalgorithm 104. In some examples, the Flight Control System that receivesthe guidance references delivered by the tracking algorithm 104 may nothave knowledge of the actual location and speed of the moving target 11.In such examples, the generated guidance references are translated fromcoordinates relative to the speed and location of the moving target 11to absolute coordinates expressed in a Global Reference System, so they(e.g., absolute coordinates) can be delivered from the trackingalgorithm 104 to the Flight Control System (FC) 112, in a manner that isfully understandable by the Flight Control System 112.

In some examples, the guidance references delivered to the FlightControl System (FC) 112 make the air vehicle 10 fly towards thereference location following a desired course.

In some examples, because the Flight Control Systems may enablereceiving inputs of certain guidance references expressed throughcertain magnitudes or parameters, the example method underlying thepresent disclosure provides that the guidance references may be given bymeans of certain parameters. These example parameters may include anycombination of one or more of at least: 1) a desired and/or thresholdpitch and/or path angle; 2) a desired and/or threshold throttle; 3) adesired and/or threshold vertical speed; 4) a desired and/or thresholdroll angle, and; 5) a particular waypoint (longitude and latitude), thiswaypoint being expressed in absolute coordinates (as explained above, insome examples, all guidance references are expressed in absolutecoordinates for the guidance references to be understood by the FlightControl System).

Thus, in some examples, after generating the reference location, thetracking algorithm 104 calculates the guidance references and deliversthe guidance references in terms of parameters (e.g., whether it be aparticular waypoint in the flying field or any other type of parameter)expressed in terms of absolute coordinates. In some examples, theguidance references are delivered to the Flight Control System 112.

In some examples, as the moving target 11 moves (continuously orintermittently) the references generated are dynamic. Therefore theexample method comprises dynamically generating new guidance referencesalong the duration of the tracking mission and/or a substantial durationof the tracking mission. Therefore, in some examples, the example methoditeratively calculates new guidance references.

In some examples, any parameters for defining the guidance referencesmay be used, provided that the resulting guidance references are basedon the reference location, and the reference location is calculated, ineach iteration of the example method, as a relative position withrespect to the current position of the moving target 11.

Therefore, in some examples, the trajectory that is followed by the airvehicle 10 changes and adapts dynamically, according to the movingtarget's position and speed.

According to an example of the method of tracking a moving target 11from an air vehicle 10, the tracking algorithm 104 calculates theguidance references based on at least two main criteria: 1) avoidingdetection of the air vehicle 10 by the moving target 11 beingtracked/followed, and; 2) avoiding loss of visual and/or sensordetection contact with the moving target 11 by the air vehicle 10.

In some examples, apart from the mentioned criteria on which thetracking algorithm 104 bases the calculation of the guidance references,the example behavior policies may also provide information onlimitations that affect low level loops of the Flight Control system(FC) 112, like pitch and roll angles. For example, there may exist aGimbal behavior policy, which imposes some limitations on the pitch androll angles and/or the course of the air vehicle 10, so that the airvehicle's own fuselage does not interrupt the line-of-sight thatconnects the air vehicle's sensors (e.g. a camera) with the movingtarget 11.

As illustrated in the example of FIG. 2, two boundaries 12, 13surrounding the position of the moving target 11 are established; as themoving target 11 moves, the two boundaries 12, 13 are dynamicallyvariable (e.g., the boundaries 12, 13 are dynamically calculated).According to an example of the method, the boundaries 12, 13 arecircular. Nevertheless, these boundaries may have any shape (e.g., oval,triangular, polygonal, concave, convex). The boundaries 12, 13, in someexamples, have the moving target 11 as their center point. However, inother examples, the moving target 11 may be offset relative to thecenter point of the boundaries 12, 13.

In some examples, in each iteration of the example method, the trackingproblem is solved, for a given altitude, in two dimensions, 2D. In someexamples, the altitude depends on each particular behavior policy, whichmay consider a terrain elevation map, to establish the particularaltitude at which the tracking problem is to be solved by the trackingalgorithm 104 in each iteration.

In some examples, the first boundary 13 establishes a minimum desireddistance of the air vehicle 10 with respect to the moving target 11 sothat the air vehicle 10 remains undetected. In some examples, the secondboundary 12 establishes a maximum desired distance of the air vehicle 10with respect to the moving target 11 so that the air vehicle 10 does notlose sensor detection contact with the moving target 11.

In some examples, the first boundary 13 is established taking intoconsideration the detectability of the air vehicle 10 by the movingtarget 11. According to a possible behavior policy, in some examples,the detectability is considered in terms of the possibility that the airvehicle 10 remains invisible by the moving target 11. For this purpose,in some examples, it is considered that the air vehicle 10 keeps out ofsight from the moving target 11 (e.g., a threshold distance, a thresholdheight, a threshold distance to deter operating noises from the aircraft10 from being heard by the moving target 11, etc.). According to anotherexample behavior policy, the detectability of the air vehicle 10 isconsidered in terms of the possibility that the air vehicle 10 remainingunheard by the moving target 11. For this purpose, the noise footprintof the air vehicle 10 may be considered, so that, as said before, theair vehicle 10 remains unheard by the moving target 11 during thetracking mission.

According to an example of the method, another example behavior policyis used to establish the second boundary 12. In some examples, thesecond boundary 12 may be established by taking into consideration thatthe sensors of the air vehicle 10 do not lose contact (e.g., sensordetection contact) with the moving target 11, e.g., line of sight and/orcapability of radar detection of the moving target 11 isensured/substantially ensured during the whole tracking mission.

In any case, in some examples, the particular behavior policy 102 is toindicate the criteria with which the first boundary 13 and the secondboundary 12 are established.

According to an example of the method, the air vehicle 10 is instructedto fly at a threshold distance from the moving target 11, said distanceconstituting a threshold and/or example location profile and/or area 14(e.g., which, as shown in the figures, may be a circle) located betweenthe first boundary 13 and the second boundary 12. The air vehicle 10 isinstructed to track the moving target 11, for example, from behind themoving target 11, in an example position (O) located at the thresholdlocation profile and/or area 14. As shown in the example of FIG. 2, theexample position (O) of the air vehicle 10 (e.g., when the dynamiccircumstances of the moving target 11 and the air vehicle 10 allow so)is a position behind the moving target 11, at a determinate altitude.Nevertheless, in some examples, an alternative behavior policy fortracking moving targets 11 may instruct the air vehicle 10 to track themoving target 11 from positions other than those behind the movingtarget 11.

Despite using circles as examples of the first boundary 13, secondboundary 12 and threshold distance, in some examples, the boundaries andthreshold distance may be any other alternative shape (e.g., polygon,square, etc.), according to alternative behavior policies. In someexamples, the alternative shape may be expressed in polar coordinateswith respect to the position of the moving target 11, such that theshape may be given as a function “r=f(theta)”, where “r” is a distancewith respect to the moving target 11 and “theta” is an angle establishedwith respect to a certain origin of the polar angular coordinate.However, the shapes and/or the position(s) of the shapes may beexpressed in any suitable way.

FIG. 2 shows an example schematic representation of the moving target 11and the air vehicle 10 (e.g., a fixed-wing air vehicle, as representedin FIG. 2). In the illustrated example, the first boundary 13, thesecond boundary 12 and the threshold location profile and/or area 14 arerepresented by the projections on the ground level of the first boundary13, the second boundary 12 and the threshold and/or example locationprofile and/or area 14 on a horizontal plane, located at the actualaltitude of the air vehicle 10. In the illustrated example, the relativealtitude (H) of the air vehicle 10 with respect to the moving target 11is also represented in FIG. 2.

As shown in the example of FIG. 2, the term “R_(min)” represents theradius of the first boundary 13; the term “R_(max)” represents theradius of the second boundary 12; the term “R_(opt)” represents theradius of the threshold and/or example location profile and/or area 14;the term “O” represents the reference and/or example position. Accordingto an example of the method presented herein, the reference and/orexample position (O) is generated as an example reference for the airvehicle 10.

As already introduced, in some examples, there may be certaincircumstances that hinder the air vehicle 10 from following the movingtarget 11 at a position behind the moving target 11. For example, if themoving target 11 is moving too slow, and the air vehicle 10 minimumgroundspeed is higher than the actual speed of the moving target 11, theair vehicle 10 may not be able to reliably perform its tracking missionby following the moving target 11 from a position directly behind themoving target 11 without risking trespassing the first boundary 13(e.g., if the air vehicle 10 passes the first boundary 13, the airvehicle 10 may be viewable by the moving target 11). Because this riskis unacceptable for most tracking missions (e.g., the risk of beingdetected by the moving target 11), the example method comprises in thoseexamples (e.g., according to a particular behavior policy) generatingtwo alternating references according to two alternating positions,represented as “O1” and “O2” in the example of FIG. 2, and instructs theair vehicle 10 to alternately fly towards either one of these twopositions (O1, O2) as long as the moving target 11 keeps moving at aspeed lower than the minimum groundspeed of the air vehicle 10, underactual wind conditions. In other words, in examples in which the movingtarget 11 is moving at a rate that is less than a threshold and/orminimum speed of the air vehicle 10, the examples disclosed hereindevelop alternate routes for the air vehicle 10 that enable the airvehicle 10 to follow the moving target in a non-direct route (e.g., notdirectly behind the moving target 11) flying side to side in a directiongenerally behind the moving target 11.

In the illustrated example, the alternating positions (O1, O2) arelocated behind the moving target 11; the vertical planes comprising thealternating positions (O1, O2) and the moving target 11 are separated byan angle “α” (as represented in FIG. 2) at each side from the verticalplane comprising the reference and/or example position (O) and themoving target 11. In some examples, the angle “α” between O and O2(e.g., thirty degrees) is different than the angle “α” between O and O1(e.g., forty five degrees).

In some examples, there may be also certain circumstances in which theactual speed of the moving target 11 is higher than the air vehicle'smaximum groundspeed, under actual wind conditions. In such examples ifthis circumstance lasts for too long, the air vehicle 10 may end uplosing sensor detection contact with the moving target 11. If the movingtarget 11 is traveling at a rate that is faster than the air vehicle 10,an estimated trajectory of the moving target 11 may be established andthe air vehicle 10 may be directed to fly along the estimatedtrajectory.

The example method comprises generating a first reference comprising thecoordinates of the reference and/or example position (O), andinstructing the air vehicle 10 to fly towards the first reference.

In some examples, because it is assumed that a reliable estimation ofthe motion of the moving target 11 is available by the air vehicle'ssensors, if it is assessed (e.g., by means of a microcontroller uniteither located onboard the air vehicle 10 or at a Control Station) atany time during the tracking mission, that the moving target's 11 actualspeed is lower than the air vehicle's 10 minimum groundspeed, underactual wind conditions, the example method comprises generating at leasttwo alternate references comprising the coordinates of either twoalternate positions (O1, O2), for example, located behind the movingtarget 11, at the reference and/or example distance constituting thethreshold and/or example location profile and/or area 14 surrounding themoving target 11.

The example method presented herein enables the tracking mission to becarried out efficiently; in this sense, the whole flight envelope of theair vehicle 10 is used for the purpose of the mission to widen thedifferent conditions (e.g., to increment the number of possiblelocations for the air vehicle 10) in which the method succeeds toprovide a valid solution (e.g., an acceptable air vehicle's locationfrom which to reliably follow and track the moving target 11). In someexamples, it is in the process of translation of the reference locationof the air vehicle 10 (e.g., relative to the moving target 11) into aguidance reference for the FC system 112 where the limitations providedby the Aircraft Performance Model (APM) 108 play their role, to enablethe air vehicle 10 not to be flown into its operational limits (e.g.,maximum speed, minimum speed, etc.).

In some examples, using the whole flight envelope of the air vehicle 10means that: 1) the tracking algorithm 104 considers the speed of the airvehicle 10, and also dynamically modifies it to substantially enable themission to be carried out in an efficient manner (e.g., a relativelyefficient manner, the most efficient manner), meaning that the airvehicle 10 can fly as slow as it is able to, if desired (e.g. the movingtarget 11 is moving very slow), or either that the air vehicle 10 canfly at faster speeds, and also proportional to how far the air vehicle10 is from the reference location, and; 2) the air vehicle performance'slimitations are considered to enable the algorithm not to violate theair vehicle's operational limits, putting the integrity of the airvehicle 10 at risk. In some examples, the operating limits of the airvehicle 10 include the minimum and/or maximum speeds and/ormaneuverability of the air vehicle 10.

According to what has been mentioned above, in some examples, aparticular behavior policy 102 may include that the guidance algorithmmodifies the air vehicle's 10 altitude, thus instructing the air vehicle10 to vary its altitude if there is an object/landmark/topography (e.g.,a mountain, a hill, a wall, etc.) that may interrupt the line-of-sightbetween the air vehicle 10 and the moving target 11.

The following is a more detailed description of the architecturecomponents depicted in FIG. 1, according to an example.

The Moving Target State Estimator Module 106 is an external componentthat is responsible for the estimation of the motion (position andvelocity) of the moving target 11 (for example, a GMT). In someexamples, it is assumed that the Moving Target State Estimator Module106 component exists and provides such information accurately, and maybe supported in computer vision and sensor data fusion and filteringalgorithms. In some examples, the Moving Target States Estimator Module106 component provides the Tracking Infrastructure Module 100 with dataon the position and velocity of the moving target 11.

The Navigation System Module 110 is also an external component that isresponsible for estimating the state vector of the air vehicle 10, forexample, a UAV position, velocity, altitude and their derivatives overtime. This estimation may be carried out by means of some fusion andfiltering algorithms (e.g., EKF approaches, Extended Kalman Filterapproaches, etc.) applied to data coming typically from onboard sensors(GNSS, accelerometers, gyroscopes, magnetometers, pressure sensors,etc.). However, the estimation may be performed in any suitable manner.In some examples, the navigation state component provides the TrackingInfrastructure Module 100 with the air vehicle's 10 state vector.

In some examples, the Flight Control System Module 112 is an externalcomponent that issues the appropriate signals to the air vehicle's 10actuators to follow the guidance references that define the mission ofthe air vehicle 10. The guidance references are, for example, generatedin the Tracking Infrastructure Module 100, therefore resulting in afully autonomous system with no need of human intervention. The guidancereferences are, for example, defined by target altitude, course andairspeed.

In some examples, the Aircraft Performance Model Module 108 is anexternal component comprising the movement characterization andmaneuverability characterization of each particular air vehicle 10. Italso may define limitations (e.g., in airspeeds, altitudes, altitudeangles, mass, etc.) and operational limits of the air vehicle 10.

In some examples, the Air Vehicle Behavior Policies Module 102 includesa group of limitations and/or constraints associated with the behaviorof the air vehicle 10 (e.g., a UAV) that affect the particular manner inwhich the moving target 11 (e.g., a GMT) is to be tracked, that is, howthe GMT Tracking problem is to be stated. In particular, in someexamples, the problem may involve the following parameters: verticaldistance, H, between the air vehicle 10 and the moving target 11,detectability distance and/or shape of the detectability area, optimumdistance and maximum distance of the air vehicle 10 with respect to themoving target 11. The following policies have been identified to cover awide range of examples.

Noise footprint policy: an example of the implementation of this policymay involve defining the detectability threshold of the air vehicle 10,being defined as the minimum distance at which the air vehicle 10 isdetectable by a moving target 11 based on its noise footprint (e.g., theability of those within the moving target 11 to hear noise generated bythe air vehicle 10). To apply such policy, in some examples, a noise mapis defined which is responsible for this policy. Regarding the statementof the moving target tracking problem stated in the TrackingInfrastructure Module 100, this policy provides constraints on thealtitude of the air vehicle 10 with respect to the moving target 11, thedetectability distance as well as in the aperture angle alpha consideredin the examples where the moving target 11 is moving slower than thecapability of the air vehicle 10. Alternatively, the detectabilitythreshold may be formed using another shape, rather than a circledetermined by the detectability distance.

Visual Identification Policy: an example of the implementation of thispolicy may involve defining the detectability threshold of the airvehicle 10 being defined as the minimum distance at which the airvehicle 10 is visually detectable by the moving target 11. Similar tothe previous policy, in regard to the statement of the moving targettracking problem, in some examples, this policy provides constraints onthe altitude of the air vehicle 10 with respect to the moving target 11,the detectability distance as well as in the aperture angle alphaconsidered in the examples where the moving target 11 is moving slowerthan the capability of the air vehicle 10.

Obstacle/Terrain elevation policy: an example of the implementation ofthis policy may involve defining altitude constraints in the statementof the moving target tracking problem that have to be fulfilled by theair vehicle 10 to keep line-of-sight with the moving target 11.

Gimbal-sensor policy: an example of the implementation of this policymay involve defining limitations on the maximum distance relative to themoving target 11 (e.g., at farther distances, the sensors of the airvehicle 10 may not be able to detect the moving target 11 with athreshold of accuracy) and also limitations regarding the altitude ofthe air vehicle 10 so that direct line-of-sight between the sensor(e.g., a camera, for example) and the moving target 11 is enabled. Inthis sense, in some examples, limits of the pan and tilt angles of thegimbal are taken into account to enable such limits to not be violatedto keep line-of-sight contact between the sensor and the moving target11. In some examples, this policy also addresses the issue of theso-called occlusion map so that the air vehicle 10 frame i does notblind the sensor at any particular altitude.

Others: in some examples, particular policy-based architectures that areflexible enough to accommodate other user policies and/or guidelinesthat affect the particular statement of the moving target trackingproblem.

According to an example of the method of tracking a moving target 11from an air vehicle 10, the Tracking Algorithm Module 104 makes a set ofassumptions in the statement of the moving target tracking problem:

In some examples, the altitude of the air vehicle 10 is a constant valueduring the resolution of the problem, and the altitude is normally theactual altitude of the air vehicle 10. Nevertheless, in some examples, aparticular policy may define a desired altitude that may be provided asa guidance reference to the flight control system. As mentioned above,between consecutive computations of the tracking algorithm 104, thereference location for the air vehicle 10 may change, and so thealtitude provided by a particular policy may also change.

In some examples, there are three different areas, comprising anygeneric shape, which define a detectability zone, a threshold and/orexample location and a non-detection zone for the air vehicle 10 totrack the moving target 11; as already introduced above, according to anexample of the method (according to an example for this policy), thesethree areas are defined by means of the horizontal distance (R_(min),R_(max), R_(opt)) between the air vehicle 10 and the moving target 11,such distances also defined by the air vehicle 10 behavior policies.

As has already been said, in some examples, a threshold and/or examplelocation for the air vehicle 10 from which to track the moving target 11is, according to an example of the method (according to an example forthis policy), a position directly behind the moving target 11 at R_(opt)distance. In case this is not possible for speed restrictions or for anyother reason, the air vehicle 10 swings around the moving target 11 at adistance R_(opt) back and forth along a sector of aperture alpha, forexample. In other words, the air vehicle 10 may follow the circumferencedefined by the area 14 to maintain within the minimum and maximumdistances from the moving target 11 (e.g., the band defined between theminimum and maximum distances).

In some examples, the air vehicle 10 avoids the detectability zone(avoids getting inside area of R_(min)) over all restrictions.

In some examples, the position and speed of the moving target 11 areknown.

In some examples, the wind speed and direction local to the air vehicle10 position are also known.

With these assumptions in mind, in some examples, the GMT Trackingproblem has three different scenarios, as also introduced above in thisdescription:

Problem case 1 example: In some examples, the moving target 11 is movingwithin the groundspeed limits of the air vehicle 10. In this example, asmentioned in the assumptions (and also according to a possible behaviorpolicy), the air vehicle 10 stays directly behind the moving target 11.Other behavior policies on which the tracking algorithm 104 may bebased, could indicate for example, as has already been said, that whenthe moving target 11 is moving within the groundspeed limits of the airvehicle 10, the air vehicle 10 varies its position relative to themoving target 11 for taking several images of the moving target 11 fromdifferent perspectives for better identifying the moving target 11.

Problem case 2 example: In some examples, the moving target 11 is movingfaster than the fastest groundspeed of the air vehicle 10. In this case,it may be unavoidable that the distance between the air vehicle 10 andthe moving target 11 increases over time until the moving target 11 iseventually lost (e.g., no longer trackable by the air vehicle 10). Insome examples, to delay such event as much as possible, the examplecourse of action in this case is to keep the air vehicle 10 at itsmaximum groundspeed in the same direction as the moving target 11 ismoving. In other words, if the moving target 11 is moving faster than amaximum speed of the air vehicle 10, the trajectory of the air vehicle10 may be set to be an estimated trajectory of the moving target 11 andthe speed of the air vehicle 10 may be set at a threshold speed (e.g., amaximum speed of the air vehicle 10).

Problem case 3 example: In some examples, the moving target 11 is movingslower than the minimum groundspeed of the air vehicle 10. According toa possible behavior policy, in some examples, the air vehicle 10 fliesat its minimum groundspeed, moving back and forth between alternatingpositions (O1, O2) at a threshold distance R_(opt). In other words, ifthe moving target 11 is moving slower than the minimum speed of the airvehicle 10, the trajectory of the air vehicle 10 may be varied to flybetween O1 and O2 while keeping a threshold distance between the airvehicle 10 and the moving target 11 to reduce or prevent detectabilityof the air vehicle 10 by the moving target 11. In some examples, otherbehavior policies could indicate that instead of moving at a thresholddistance from the moving target 11, the air vehicle 10 moves around themoving target 11 without trespassing a contour surrounding the movingtarget 11 (e.g., the first boundary 13), the contour having anypredetermined shape established by each particular behavior policy.

The following is a description of an example of the tracking algorithm104, performing the method of the present disclosure. The blocks of thetracking algorithm 104 are schematically depicted in FIG. 3. Whendescribing the processes of the algorithm, reference is to be madehereinafter to FIGS. 4, 5 and 6.

In some examples, the tracking algorithm 104 generates guidancereferences to track the moving target 11 (e.g., a GMT) is based instating the problem of the relative motion of the air vehicle 10 (pointA in FIGS. 4, 5 and 6), which has been represented as a fixed-wing airvehicle 10 by way of example, with respect to the moving target 11(point G in FIGS. 4, 5 and 6). In some examples, the tracking algorithm104 calculates the relative speed vector of A (e.g., the location of theair vehicle 10) with respect to G (e.g., the location of the movingtarget 11).

The particular example algorithm shown in FIG. 3 is made up of 9processes. According to example illustrated in FIG. 3, the algorithmperforming the method comprises:

In this example, the method begins with stating the problem in planarcoordinates and/or projecting into planar coordinates both thecoordinates of the air vehicle 10 and the coordinates of the movingtarget 11 (block 1). The projection may be orthomorphic (conformal) topreserve angles. In this example, the problem is then stated in theCartesian reference system defined by:

Setting the coordinates origin in the actual location of the movingtarget 11;

Establishing the Y coordinated axis in the direction of motion of themoving target 11;

Establishing the X coordinated axis so that the resulting coordinates'system is right-handed oriented.

In this example, the method defines a desired location and/or defines areference location of the air vehicle 10 which, according to thedifferent contemplated scenarios mentioned above, may either be:

Position O in problem case 1;

The actual position of the air vehicle in problem case 2;

Either Position O, position O1 or position O2 in problem case 3 (block2).

For the sake of a clear representation, in this example, FIGS. 5 and 6represent the problem case 1, e.g., the intended air vehicle's locationis defined by position O.

In this example, the method estimates errors (or divergences) withrespect to the reference location of the air vehicle 10 defined in theprevious process (block 3). Two different errors are defined andrepresented in FIG. 5:Radial error: ε_(r) =R _(opt) −|GA|.  Equation 1

In this example, Equation 1 represents how far the air vehicle 10 isfrom the reference location, in the normal direction, wherein for thepurposes of the radial error, the reference location is given by itsdistance to the moving target 11, R_(opt) (the radial distance among themoving target 11 and the reference location).Tangential error: ε_(s)=ε_(θ) *R _(opt), whereε_(θ)=θ_(A)−θ_(o).  Equation 2

In this example, the tangential error defined by equation 2 representshow far the air vehicle 10 is from the reference location in thetangential direction.

In this example, the method calculates the desired course relative tothe moving target 11 and/or calculates 4 the desired direction of therelative motion of the air vehicle 10 with respect to the moving target11 (block 4). In some examples, the computed errors are expressed in ann−t (normal−tangential) reference system centered in the air vehicle 10(Reference system A, u_(n), u_(t)), the direction of the relative speedvector of the air vehicle 10 with respect to the moving target 11 isdefined by the Equation 3 (see FIG. 5):

$\begin{matrix}{\beta_{AG} = {{atan}\frac{\epsilon_{r}}{ɛ_{s}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

In some examples, even though the air vehicle 10 may be placed at adistance R_(opt), if the air vehicle 10 is in front of the moving target11 (instead of behind the moving target 11), the radial error is null(ε_(r)=0) while the tangential error is very high (ε_(s)=π*R_(opt)); tocorrect such error, in some examples, the air vehicle 10 moves quicklyin the tangential direction, avoiding getting into the detectabilityzone.

In this example, the method calculates the intended ground speed of theair vehicle 10 relative to the moving target 11 and/or calculates 5 themagnitude of the relative speed vector of the air vehicle 10 withrespect to the moving target 11 (block 5).

In some examples, a proportional (proportional to the errors calculated)response of the air vehicle 10 may eventually be desired, so that, ifthe air vehicle 10 is far from the reference location (in either theradial direction, the tangential direction or both) the air vehicle 10reacts faster than in the case the air vehicle 10 is closer to thereference location (in which case, in some examples, the desiredrelative speed is minimized to optimize the block). In some examples, aproportional response behavior fits very well in a PID approach fromclassic control theory, for example.

The magnitude of the relative speed vector of the air vehicle 10 withrespect to the moving target 11 may be defined by Equation 4:

$\begin{matrix}{V_{AG} = {\left( {{{kp}_{r}*ɛ_{r}} + {{kd}_{r}*\frac{d\; ɛ_{r}}{dt}} + {{ki}_{r}*{\int_{t_{0}}^{t_{1}}{ɛ_{r}*{dt}}}}} \right) + \left( {{{kp}_{s}*ɛ_{s}} + {{kd}_{s}*\frac{d\; ɛ_{s}}{dt}} + {{ki}_{s}*{\int_{t_{0}}^{t_{1}}{ɛ_{s}*{dt}}}}} \right)}} & {{Equation}\mspace{14mu} 4}\end{matrix}$wherein:

Kp_(r) and Kp_(s) are proportional to the radial and tangential errorsrespectively;

Kd_(r) and Kd_(s) are proportional to the variation over time of theradial and tangential errors respectively;

Ki_(r) and Ki_(s) are proportional to the integration during aparticular time interval of the radial and tangential errorsrespectively.

In some examples, even though the magnitude of the relative speed vectorof the air vehicle 10 with respect to the moving target 11 has beendefined by means of full PID's, it is up to the user to fullycharacterize them (e.g., the magnitude of the relative speed vector ofthe air vehicle 10); for example, a fully proportional response may begiven by 2 parameters (Kp_(r) and Kp_(s)).

In some examples, the magnitude of the relative speed vector of the airvehicle 10 with respect to the moving target 11 may be calculatedaccording to any existing control theory (e.g., other than a PID-basedcontrol theory, a proportional integral derivative controller) whichcombine one or more of the estimated errors.

In this example, the method calculates the absolute groundspeed, V_(A),of the air vehicle 10 (block 6). The absolute groundspeed of the airvehicle 10 can be determined and/or achieved by vector algebra, as thegroundspeed of the moving target 11 is assumed to be known. Vector V_(A)may be obtained from Equation 5:{right arrow over (V _(A))}={right arrow over (V _(AG))}+{right arrowover (V _(G))}  Equation 5:wherein:

V_(A) is the absolute groundspeed of the air vehicle 10;

V_(AG) is the relative speed of the air vehicle 10 with respect to themoving target 11;

V_(G) is the groundspeed of the moving target 11.

In this example, the method estimates and/or calculates, the airspeed ofthe air vehicle 10 (block 7). In some examples, so far all speedmagnitudes involved in the problem are groundspeed, which may not be acommon magnitude used in aviation (as all vehicle performances andlimitations depend rather on airspeed). Therefore, the resultingairspeed may be calculated, again with vector algebra, taking intoaccount the local wind conditions. Equation 6 may be used to determinethe resulting airspeed:{right arrow over (V _(WA))}={right arrow over (V _(W))}+{right arrowover (V _(A))}  Equation 6:wherein:

V_(WA) is the airspeed of the air vehicle 10;

V_(W) is the wind vector at the air vehicle's position;

V_(A) is the absolute groundspeed vector of the air vehicle 10,calculated in the previous block.

In this example, the method checks performance limitations of the airvehicle 10 and/or checks performance limitations (block 8 a).

In some examples, if the airspeed previously calculated falls below thestall speed or above the maximum airspeed values (e.g., these values aretaken with a predetermined and/or threshold tolerance range, tocompensate a possible uncertainty in wind estimation, so as to deter theair vehicle 10 accidentally reaching its stall speed and/or maximumairspeed limits) of the air vehicle 10 (e.g., depending on theparticular local wind field as well as the aforementioned PIDparameters). For example, if the desired and/or threshold airspeed ofthe air vehicle 10 is outside of the operating speeds (e.g., a maximumspeed, a minimum speed,) and/or outside of other operating thresholds ofthe air vehicle 10, then the airspeed may be adjusted to such values(e.g., adjust the desired air speed to be within the operatingcapabilities of the air vehicle 10). In the illustrated example,adjusting the airspeed values involving recalculating the groundspeedvector of the air vehicle 10 (block 8 b), recalculating course (block 8c), so that the groundspeed vector points towards the referencelocation, and returning to (block 7), and iterating the block until avalid solution is found.

In this example, the method feeds the flight control system and/orprovides a guidance reference to the air vehicle 10 (block 9). In someexamples, these guidance references may include at least three types ofreferences or magnitudes that may be provided to the air vehicle 10Flight Control System (FC) 112. These references, which may define howthe air vehicle 10 is to be operated, are based on common magnitudesused in Flight Control Systems as guidance references:

In some examples, a desired and/or threshold course (given by thedirection of the absolute groundspeed vector of the air vehicle 10) maybe provided according to any combination of parameters (e.g., heading orbearing in either magnetic or true flavors may be provided; also theabsolute coordinates of the reference location may be provided, bearingin mind that the reference location is continuously recalculated in eachiteration of the method, as the moving target 11 and/or the air vehicle10 keep moving);

A desired airspeed (e.g., given by the module of the airspeed of the airvehicle 10, or any derivative magnitude);

A desired altitude, taking into account the air vehicle 10 behaviorpolicies.

In some examples, the guidance references are provided to the FC system112, with respect to the moving target's state vector (estimatedposition, speed, etc.). Although the guidance reference may include adesired course (e.g., it may not specify a desired location, but adesired course), in some examples, a reference location of the airvehicle 10 with respect to the moving target's 11 estimated location iscalculated first, to generate a guidance reference as a desired coursepointing to that calculated reference location.

In some examples, the guidance references are continuously re-calculatedand given to the FC system 112 of the air vehicle 10 so that the airvehicle 10 dynamically adapts its motion to the tracking missionrequirements (e.g., to the moving target's 11 state vector).

In some examples, there are also some limitations to be provided to theFlight Control System 112; such is the case of those provided by theGimbal-sensor policy that define an altitude limitations map (e.g.,pairs of pitch/roll limits) that have to be taken into account by theFlight Control System 112 to enable the particular sensor to be in theline-of-sight with the moving target 11, e.g., that the air vehicle'sfuselage does not interrupt the line-of-sight connecting each sensorwith the moving target 11.

Further, examples of the methods disclosed may include a computerprogrammed to operate in accordance with the method described herein.The computer may be associated with a conventional flight simulator. Inturn, the computer may include a processor and a memory for a computerprogram that, when executed, causes the processor to operate inaccordance with the method described herein. The computer program mayalso be embodied in a computer readable medium having stored therein thecomputer program.

In some examples, to solve the inconveniences and address the issuesmentioned above, an example method of tracking a moving target from anair vehicle 10 is presented herewith.

The example method of tracking a moving target from an air vehicle 10includes determining an estimated location and speed of at least onemoving target and instructing an air vehicle 10 to follow the movingtarget.

The example method of tracking a moving target from an air vehicle 10includes: determining a detectability zone surrounding the movingtarget; calculating at least one reference location for the air vehicle10, wherein the reference location is a location relative to theestimated location and/or speed of the moving target, wherein thereference location is placed out of the detectability zone; generatingat least one guidance reference to command the air vehicle 10 fortracking the moving target, wherein the guidance reference comprises atleast a desired course to fly towards the calculated reference location;a desired speed to fly towards the calculated reference location; adesired flying altitude, and; instructing the air vehicle to flyaccording to the generated guidance reference.

In some examples, the determination of the detectability zone, thecalculation of a reference location and the generation of at least oneguidance reference are performed according to at least one predeterminedbehavior policy of the air vehicle.

According to an example of the method, the desired speed is calculatedbased on the divergence existing among actual/current location of theair vehicle and the calculated reference location of the air vehicle.

The mentioned divergence existing among the actual location of the airvehicle and the reference location of the air vehicle may be calculatedbased on any combination of one or more of at least angular deviationexisting between a first ground imaginary line which is a projection ona horizontal plane of a first imaginary line which links the movingtarget with the air vehicle; and a second ground imaginary line which isa projection on the horizontal plane of a second imaginary line whichlinks the moving target with the at least one reference location of theair vehicle, and; radial deviation existing between: a circumferencecentered in the moving target and passing along a projection in thehorizontal plane of the actual location of the air vehicle; and acircumference centered in the moving target and passing along aprojection in the horizontal plane of the reference location of the airvehicle.

In some examples, the desired speed at which the air vehicle fliestowards the reference location may be calculated according to anycombination of one or more of at least proportional, integral andderivative criteria, which mathematically combine any combination of oneor more of at least the angular deviation and the radial deviation.

According to an example, the desired speed to fly towards the referencelocation is calculated based on wind velocity local to the actuallocation of the air vehicle.

In some examples, a predetermined behavior policy indicates thedetectability zone is a circle in a horizontal plane, centered in themoving target. However, the detectability zone may be any shape and themoving target may be positioned in any location within the detectabilityzone.

Also, according to an example behavior policy, the detectability zonemay be based on any combination of one or more of at least probabilitythat the moving target detects sound emitted by the air vehicle, whereinthis probability is based on features of the air vehicle, and;probability that the moving target visually detects the air vehicle,wherein this probability is based on features of the air vehicle.

According to an example of the method, a predetermined behavior policyfor calculating the reference location is based on placing the referencelocation among a first boundary, which is a limit for the detectabilityzone, and a second boundary, which is a limit beyond which the airvehicle is not likely to detect the moving target, wherein thisprobability is based on features of the air vehicle. In other words, theair vehicle flies between a minimum and maximum threshold distance fromthe moving target by flying directly behind the moving target or flyingside to side (e.g., side to side within a circumferential band and/orzone) within a zone defined by the minimum and maximum thresholddistances.

According to an example behavior policy, the first boundary and thesecond boundary have the shapes of circles in a horizontal plane, thecircles centered in the moving target.

Also according to an example behavior policy, the reference location ofthe air vehicle is placed at an altitude which is adjusted depending onterrain characteristics, so that if an obstacle hinders detection of themobile target from the air vehicle, the altitude of the referencelocation is accordingly increased or decreased.

In some examples, the method includes, according to an example thereof,assessing whether the estimated speed of the moving target is greaterthan a maximum groundspeed (speed with respect to the ground) allowedfor the air vehicle, under actual wind conditions (under the windconditions at the time and location where the air vehicle is currentlyflying); is less than minimum groundspeed allowed for the air vehicle,under actual wind conditions; or falls between the minimum groundspeedallowed for the air vehicle and the maximum groundspeed allowed for theair vehicle (or is equal to one of these limits), under actual windconditions.

According to an example of the method, if the speed of the moving targetis lower than the minimum groundspeed of the air vehicle, the methodincludes generating at least: a first guidance reference and a secondguidance reference, wherein the first guidance reference includes atleast one first reference location for the air vehicle and the secondguidance reference includes at least one second reference location forthe air vehicle, wherein the first reference location and the secondreference location are placed out of the detectability zone, and;instructing the air vehicle to alternately fly toward the firstreference location and toward the second reference location, accordingto the generated first guidance reference and second guidance reference.

According to an example of the method, if the speed of the moving targetis greater than the maximum groundspeed of the air vehicle, the methodincludes generating at least one guidance reference comprising at leastone reference location for the air vehicle, wherein the referencelocation is placed out of the detectability zone and wherein the methodincludes instructing the air vehicle to fly toward the referencelocation at the maximum groundspeed allowed for the air vehicle, underactual wind conditions.

Lastly, according to an example of the method, if the speed of themoving target is equal to the minimum groundspeed for the air vehicle orfalls between the minimum groundspeed allowed for the air vehicle andthe maximum groundspeed allowed for the air vehicle, under actual windconditions, the method includes generating at least one guidancereference comprising at least one reference location for the airvehicle, wherein the reference location for the air vehicle is placedout of the detectability zone and right behind and/or adjacent to themoving target.

The present disclosure also refers to a computer readable mediumincluding instructions for carrying out the method described above.

The present disclosure also refers to an example system for tracking amoving target from an air vehicle, wherein the system includes aTracking Infrastructure Module which in turn comprises an Air VehicleBehavior Policies Module and Tracking Algorithm Module 104. The TrackingAlgorithm Module 104 is configured to perform the method describedabove, according to behavior policies stored in the Air Vehicle BehaviorPolicies Module.

The main challenges that may be addressed in ISR missions are: theestimation of the moving target motion (position, velocity andacceleration) and the actual guidance of the air vehicle.

Provided that a reliable estimation of the moving target motion isavailable, in some examples, the issues that are addressed when tacklingthe problem of tracking a moving target from an air vehicle include:minimum detectability by the moving target; avoiding sensor blind spotsthat could lead the air vehicle to lose visual contact with the movingtarget to be tracked; managing variable weather conditions; consideringboth moving target dynamics and air vehicle motion characteristics, and;considering terrain characteristics (obstacles, terrain elevation,etc.). An example method of tracking a moving target (11) from an airvehicle (10) comprising determining an estimated location and speed ofat least one moving target (11) and instructing an air vehicle (10) tofollow the moving target (11). The example method comprises: determininga detectability zone surrounding the moving target (11); calculating atleast one reference location for the air vehicle 10, relative to theestimated location of the moving target (11), wherein the referencelocation is placed out of the detectability zone; generating at leastone guidance reference to command the air vehicle (10) for tracking themoving target (11), wherein the guidance reference comprises anycombination of one or more of at least: a desired course to fly towardsthe calculated reference location; a desired speed to fly towards thecalculated reference location; a desired flying altitude, and;instructing the air vehicle (10) to fly according to the generatedguidance reference, wherein the determination of the detectability zone,the calculation of a reference location and the generation of at leastone guidance reference are performed according to at least onepredetermined behavior policy of the air vehicle (10).

In some examples, the desired speed is calculated based on thedivergence existing among actual location of the air vehicle (10) andthe reference location of the air vehicle (10). In some examples, thedivergence existing among the actual location of the air vehicle (10)and the at least one reference location of the air vehicle (10) iscalculated based on any combination of one or more of at least: angulardeviation existing between: a first ground imaginary line which is aprojection on a horizontal plane of a first imaginary line which linksthe moving target (11) with the air vehicle (10); and a second groundimaginary line which is a projection on the horizontal plane of a secondimaginary line which links the moving target (11) with the at least onereference location of the air vehicle (10), and; radial deviationexisting between: a circumference centered in the moving target (11) andpassing along a projection in the horizontal plane of the actuallocation of the air vehicle (10); and a circumference centered in themoving target (11) and passing along a projection in the horizontalplane of the at least one reference location of the air vehicle (10).

In some examples, the desired speed at which the air vehicle (10) fliestowards the at least one reference location is calculated according toany combination of one or more of at least proportional, integral andderivative criteria, which mathematically combine any combination of oneor more of at least the angular deviation and the radial deviation. Insome examples, the desired speed to fly towards the reference locationis calculated based on wind velocity local to the actual location of theair vehicle (10). In some examples, according to a predeterminedbehavior policy, the detectability zone is a circle in a horizontalplane, centered in the moving target (11). In some examples, apredetermined behavior policy for determining the detectability zone isbased on any combination of one or more of at least: probability thatthe moving target (11) detects sound emitted by the air vehicle (10),wherein this probability is based on features of the air vehicle (10),and; probability that the moving target (11) visually detects the airvehicle (10), wherein this probability is based on features of the airvehicle (10).

In some examples, a predetermined behavior policy for calculating thereference location is based on placing the reference location among afirst boundary (13), which is a limit for the detectability zone, and asecond boundary (12), which is a limit beyond which the air vehicle (10)is not likely to detect the moving target (11), wherein this probabilityis based on features of the air vehicle (10). In some examples,according to a predetermined behavior policy, the first boundary (13)and the second boundary (12) have the shapes of circumferences in ahorizontal plane, the circumferences centered in the moving target (11).In some examples, according to a predetermined behavior policy, thereference location of the air vehicle (10) is placed at an altitudewhich is adjusted depending on terrain characteristics, so that if therecomes up an obstacle which hinders detection of the mobile target (11)from the air vehicle (10), the altitude of the reference location isaccordingly raised or lowered.

In some examples, assessing whether the estimated speed of the movingtarget (11): is higher than maximum groundspeed allowed for the airvehicle (10), under actual wind conditions; is lower than minimumgroundspeed allowed for the air vehicle (10), under actual windconditions; or is equal or falls between the minimum groundspeed allowedfor the air vehicle (10) and the maximum groundspeed allowed for the airvehicle (10), under actual wind conditions. In some examples, if thespeed of the moving target (11) is lower than the minimum groundspeed ofthe air vehicle (10), under actual wind conditions, the methodcomprises: generating at least: a first guidance reference and a secondguidance reference, wherein the first guidance reference comprises atleast one first reference location for the air vehicle (10) and thesecond guidance reference comprises at least one second referencelocation for the air vehicle (10), wherein the first reference locationand the second reference location are placed out of the detectabilityzone, and; instructing the air vehicle (10) to alternately fly towardsthe at least one first reference location and towards the at least onesecond reference location, according to the generated first guidancereference and second guidance reference.

In some examples, if the speed of the moving target (11) is higher thanthe maximum groundspeed of the air vehicle (10), under actual windconditions, the method comprises generating at least one guidancereference comprising at least one reference location for the air vehicle(10), wherein the reference location is placed out of the detectabilityzone and wherein the method comprises instructing the air vehicle (10)to fly towards the reference location at the maximum groundspeed allowedfor the air vehicle (10), under actual wind conditions. In someexamples, if the speed of the moving target (11) is equal or fallsbetween the minimum groundspeed allowed for the air vehicle (10) and themaximum groundspeed allowed for the air vehicle (10), under actual windconditions, the method comprises generating at least one guidancereference comprising at least one reference location for the air vehicle(10), wherein the reference location for the air vehicle is placed outof the detectability zone and right behind the moving target (11).

An example system for tracking a moving target (11) from an air vehicle(10) includes a Tracking Infrastructure Module (100) which in turncomprises an Air Vehicle Behavior Policies Module (102) and TrackingAlgorithm Module (104), wherein the Tracking Algorithm Module (104) isconfigured to perform the method, according to behavior policies storedin the Air Vehicle Behavior Policies Module (102).

An example computer readable medium for tracking a moving target from anair vehicle, the computer-readable storage medium being non-transitoryand having computer-readable program code portions stored therein thatin response to execution by a processor, cause an apparatus to at least:determine a detectability zone surrounding the moving target; calculateat least one reference location for the air vehicle, relative to theestimated location of the moving target, wherein the reference locationis placed out of the detectability zone; generate at least one guidancereference to command the air vehicle for tracking the moving target,wherein the guidance reference comprises any combination of one or moreof at least: a desired course to fly towards the calculated referencelocation; a desired speed to fly towards the calculated referencelocation; a desired flying altitude, and; instruct the air vehicle tofly according to the generated guidance reference wherein thedetermination of the detectability zone, the calculation of a referencelocation and the generation of at least one guidance reference areperformed according to at least one predetermined behavior policy of theair vehicle.

In some examples, the desired speed is calculated based on thedivergence existing among actual location of the air vehicle and thereference location of the air vehicle. In some examples, the divergenceexisting between the actual location of the air vehicle and the at leastone reference location of the air vehicle is calculated based on atleast one of: angular deviation existing between: a first groundimaginary line which is a projection on a horizontal plane of a firstimaginary line which links the moving target with the air vehicle; asecond ground imaginary line which is a projection on the horizontalplane of a second imaginary line which links the moving target with theat least one reference location of the air vehicle; radial deviationexisting between: a circumference centered in the moving target andpassing along a projection in the horizontal plane of the actuallocation of the air vehicle; or a circumference centered in the movingtarget and passing along a projection in the horizontal plane of the atleast one reference location of the air vehicle.

In some examples, the desired speed at which the air vehicle fliestowards the at least one reference location is calculated according toany combination of one or more of at least proportional, integral andderivative criteria, which mathematically combine any combination of oneor more of at least the angular deviation and the radial deviation. Insome examples, the desired speed to fly towards the reference locationis calculated based on wind velocity local to the actual location of theair vehicle. In some examples, a predetermined behavior policy fordetermining the detectability zone is based on any combination of one ormore of at least one of the moving target detects sound emitted by theair vehicle, wherein this probability is based on features of the airvehicle, and; probability that the moving target visually detects theair vehicle, wherein this probability is based on features of the airvehicle.

An example system for tracking a moving target from an air vehiclewherein the system comprises a Tracking Infrastructure Module which inturn comprises an Air Vehicle Behavior Policies Module and TrackingAlgorithm Module, wherein the system comprising: a processor and amemory storing executable instructions that, in response to execution bythe processor, cause the Tracking Algorithm Module to at least:determine a detectability zone surrounding the moving target; calculateat least one reference location for the air vehicle, relative to theestimated location of the moving target, wherein the reference locationis placed out of the detectability zone; generate at least one guidancereference to command the air vehicle for tracking the moving target,wherein the guidance reference comprises any combination of one or moreof at least: a desired course to fly towards the calculated referencelocation; a desired speed to fly towards the calculated referencelocation; a desired flying altitude, and; instruct the air vehicle tofly according to the generated guidance reference; wherein thedetermination of the detectability zone, the calculation of a referencelocation and the generation of at least one guidance reference areperformed according to at least one predetermined behavior policy of theair vehicle.

In some examples, the desired speed at which the air vehicle fliestowards the at least one reference location is calculated according toany combination of one or more of at least proportional, integral andderivative criteria, which mathematically combine any combination of oneor more of at least the angular deviation and the radial deviation. Insome examples, the desired speed to fly towards the reference locationis calculated based on wind velocity local to the actual location of theair vehicle. In some examples, a predetermined behavior policy fordetermining the detectability zone is based on any combination of one ormore of at least one of the moving target detects sound emitted by theair vehicle, wherein this probability is based on features of the airvehicle, and; probability that the moving target visually detects theair vehicle, wherein this probability is based on features of the airvehicle.

An example method of tracking a moving target from an air vehiclecomprising determining an estimated location and speed of a movingtarget and instructing an air vehicle to follow the moving target,wherein the method further comprises: a) determining a detectabilityzone surrounding the moving target; b) calculating at least onereference location for the air vehicle; c) generating at least oneguidance reference to command the air vehicle for tracking the movingtarget, wherein the guidance reference comprises any combination of oneor more of at least: c.1) a desired course; c.2) a desired speed; c.3) adesired flying altitude; and wherein the method further comprises: d)instructing the air vehicle to fly according to the generated guidancereference; wherein the determination of the detectability zone, thecalculation of a reference location and the generation of the guidancereference are performed according to at least one behavior policy.

The description of the different examples have been presented forpurposes of illustration and description, and is not intended to beexhaustive or limited to the examples in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art. Furthermore, different examples may provide differentadvantages as compared to other advantageous examples.

The invention claimed is:
 1. A system comprising: an air vehicleincluding: a moving target state estimator to determine at least one ofan estimated speed or an estimated location of a moving target; atracking infrastructure to: determine a detectability zone surroundingthe moving target based on at least one of the estimated speed or theestimated location of the moving target; and generate a guidancereference to command the air vehicle to move towards a referencelocation, the reference location based on the estimated location; and aflight control system to cause the air vehicle to follow the movingtarget outside of the detectability zone based on the guidancereference.
 2. The system of claim 1, wherein the tracking infrastructureis to generate the guidance reference based on at least one of avoidingdetection of the air vehicle by the moving target or avoiding loss of atleast one of visual or sensor detection contact with the moving targetby the air vehicle.
 3. The system of claim 1, wherein the trackinginfrastructure is to determine the reference location by: determiningcoordinates based on a reference system centered on the moving target;and generating a position vector with respect to the moving target basedon the coordinates to place the reference location outside of thedetectability zone, the reference location relative to the estimatedlocation of the moving target.
 4. The system of claim 1, wherein theguidance reference includes at least one of an altitude the air vehicleis to use when flying towards the reference location, a course the airvehicle is to use when flying towards the reference location, or a speedthe air vehicle is to use when flying towards the reference location,the speed corresponding to an airspeed or a groundspeed of the airvehicle.
 5. The system of claim 1, wherein the tracking infrastructureis to determine the detectability zone based on at least one of a firstprobability that sound emitted by the air vehicle is detectable at themoving target or a second probability that the air vehicle is visuallydetectable by the moving target.
 6. The system of claim 1, wherein thedetectability zone is a first detectability zone and the trackinginfrastructure is to: determine a second detectability zone associatedwith a boundary beyond which the air vehicle is unlikely to detect themoving target; and determine the reference location of the air vehiclerelative to the estimated location of the moving target, the referencelocation based on the first detectability zone and the seconddetectability zone.
 7. The system of claim 1, wherein the guidancereference includes a speed the air vehicle is to fly when following themoving target and the tracking infrastructure is to determine the speedby: determining an angular deviation between an actual location of theair vehicle and the reference location of the air vehicle; determining aradial deviation between the actual location of the air vehicle and thereference location of the air vehicle; determining a first differencebetween the angular deviation and the radial deviation; and determiningthe speed based on a second difference between the estimated location ofthe moving target and the reference location, the second differencebased on the first difference.
 8. The system of claim 7, wherein theangular deviation is associated with the angular deviation between afirst projection on a horizontal plane that links the moving target andthe air vehicle and a second projection on the horizontal plane thatlinks the moving target with the reference location.
 9. The system ofclaim 8, wherein the radial deviation is associated with the radialdeviation between a first circumference centered on the moving targetand passing along the first projection and a second circumferencecentered on the moving target and passing along the second projection.10. An apparatus comprising: a moving target state estimator todetermine at least one of an estimated speed or an estimated location ofa moving target; and a tracking infrastructure to: determine adetectability zone surrounding the moving target based on at least oneof the estimated speed or the estimated location of the moving target;and cause an air vehicle to follow the moving target outside of thedetectability zone based on a guidance reference, the guidance referenceto command the air vehicle to move towards a reference location, thereference location based on the estimated location.
 11. The apparatus ofclaim 10, wherein the tracking infrastructure is to generate theguidance reference based on at least one of avoiding detection of theair vehicle by the moving target or avoiding loss of at least one ofvisual or sensor detection contact with the moving target by the airvehicle.
 12. The apparatus of claim 10, wherein the trackinginfrastructure is to determine the reference location by: determiningcoordinates based on a reference system centered on the moving target;and generating a position vector with respect to the moving target basedon the coordinates to place the reference location outside of thedetectability zone, the reference location relative to the estimatedlocation of the moving target.
 13. The apparatus of claim 10, whereinthe guidance reference includes at least one of an altitude the airvehicle is to use when flying towards the reference location, a coursethe air vehicle is to use when flying towards the reference location, ora speed the air vehicle is to use when flying towards the referencelocation, the speed corresponding to an airspeed or a groundspeed of theair vehicle.
 14. The apparatus of claim 10, wherein the trackinginfrastructure is to determine the detectability zone based on at leastone of a first probability that sound emitted by the air vehicle isdetectable at the moving target or a second probability that the airvehicle is visually detectable by the moving target.
 15. The apparatusof claim 10, wherein the detectability zone is a first detectabilityzone and the tracking infrastructure is to: determine a seconddetectability zone associated with a boundary beyond which the airvehicle is unlikely to detect the moving target; and determine thereference location of the air vehicle relative to the estimated locationof the moving target, the reference location based on the firstdetectability zone and the second detectability zone.
 16. The apparatusof claim 10, wherein the guidance reference includes a speed the airvehicle is to fly when following the moving target and the trackinginfrastructure is to determine the speed by: determining an angulardeviation between an actual location of the air vehicle and thereference location of the air vehicle; determining a radial deviationbetween the actual location of the air vehicle and the referencelocation of the air vehicle; determining a first difference between theangular deviation and the radial deviation; and determining the speedbased on a second difference between the estimated location of themoving target and the reference location, the second difference based onthe first difference.
 17. An apparatus comprising: memory storinginstructions; and at least one processor to execute the instructions tocause the at least one processor to: determine at least one of anestimated speed or an estimated location of a moving target; determine adetectability zone surrounding the moving target based on at least oneof the estimated speed or the estimated location of the moving target;and cause an air vehicle to follow the moving target outside of thedetectability zone based on a guidance reference, the guidance referenceto command the air vehicle to move towards a reference location, thereference location based on the estimated location.
 18. The apparatus ofclaim 17, wherein the guidance reference is based on at least one ofavoiding detection of the air vehicle by the moving target or avoidingloss of at least one of visual or sensor detection contact with themoving target by the air vehicle.
 19. The apparatus of claim 17, furtherincluding instructions which, when executed, cause the at least oneprocessor to: determine coordinates based on a reference system centeredon the moving target; and generate a position vector with respect to themoving target based on the coordinates to place the reference locationoutside of the detectability zone, the reference location relative tothe estimated location of the moving target.
 20. The apparatus of claim17, wherein the guidance reference includes at least one of an altitudethe air vehicle is to use when flying towards the reference location, acourse the air vehicle is to use when flying towards the referencelocation, or a speed the air vehicle is to use when flying towards thereference location, the speed corresponding to an airspeed or agroundspeed of the air vehicle.
 21. The apparatus of claim 17, whereindetermining the detectability zone is based on at least one of a firstprobability that sound emitted by the air vehicle is detectable at themoving target or a second probability that the air vehicle is visuallydetectable by the moving target.
 22. The apparatus of claim 17, whereinthe detectability zone is a first detectability zone and furtherincluding instructions which, when executed, cause the at least oneprocessor to: determine a second detectability zone associated with aboundary beyond which the air vehicle is unlikely to detect the movingtarget; and determine the reference location of the air vehicle relativeto the estimated location of the moving target, the reference locationbased on the first detectability zone and the second detectability zone.23. The apparatus of claim 17, wherein the guidance reference includes aspeed the air vehicle is to fly when following the moving target andfurther including instructions which, when executed, cause the at leastone processor to: determine an angular deviation between an actuallocation of the air vehicle and the reference location of the airvehicle; determine a radial deviation between the actual location of theair vehicle and the reference location of the air vehicle; determine afirst difference between the angular deviation and the radial deviation;and determine the speed based on a second difference between theestimated location of the moving target and the reference location, thesecond difference based on the first difference.